Determining surface structure and stability of epsilon-Fe2C, chi-Fe5C2, theta-Fe3C and Fe4C phases under carburization environment from combined DFT and atomistic thermodynamic studies

The chemical-physical environment around iron based FTS catalysts under working conditions is used to estimate the influences of carbon containing gases on the surface structures and stability of epsilon-Fe2C, chi-Fe5C2, theta-Fe3C and Fe4C from combined density functional theory and atomistic-thermodynamic studies. Higher carbon content gas has higher carburization ability; while higher temperature and lower pressure as well as higher H-2/CO ratio can suppress carburization ability. Under wide ranging gas environment, epsilon-Fe2C, chi-Fe5C2 and theta-Fe3C have different morphologies, and the most stable non-stoichiometric termination changes from carbon-poor to carbon-rich (varying surface Fe/C ratio) upon the increase in DmC. The most stable surfaces of these carbides have similar surface bonding pattern, and their surface properties are related to some common phenomena of iron based catalysts. For these facets, chi-Fe5C2-(100)-2.25 is most favored for CO adsorption and CH4 formation, followed by theta-Fe3C-(010)-2.33, epsilon-Fe2C-(1 (2) over bar1)-2.00 and Fe4C-(100)-3.00, in line with surface work function and the charge of the surface carbon atoms.